The monomer-dimer equilibria of liquid aluminum alkyls : IV. Triethylaluminum in mesitylene

Abstract

The monomer-dimer equilibrium of triethylaluminum (TEA) in mesitylene solution has been studied by measuring heats of dilution. The value obtained for the heat of dissociation of one mole of TEA dimer in mesitylene, 13.3 1 kcal, is significantly smaller than the corresponding value in n-hexadecane (16.9 3 kcal). The difference is ascribed primarily to the (exothermic) complexation of mesitylene with TEA monomer. Degrees of dissociation of TEA in mesitylene at various mole fractions are tabulated over a wide temperature range and compared with corresponding values in aliphatic (saturated) hydrocarbons. The results indicate that TEA is about eight times as dissociated in mesitylene as in an aliphatic hydrocarbon at −50°C. The ratio decreases with rising temperature and becomes unity at about 170° C. The results have been extended to trimethylaluminum (TMA) and to other aromatic solvents. The estimated value for the heat of dissociation of TMA in toluene (15.6±1.4 kcal· (mole of dimer) −1) is consistent with the literature value (15.4±2.0 kcal·mole −1) for the activation energy for the bridge—terminal exchange of methyl groups. This supports the literature contention that the exchange in toluene solution occurs via dissociation of dimeric molecules to monomers as the rate determining step. Since the literature value for the activation energy for exchange in cyclopentane solution (15.6±0.2 kcal·mole −1) is well below the heat of dissociation of TMA in cyclopentane (19.4 kcal·mole −1), there is a distinct possibility that the exchange mechanism is different in the two solvents.